Nuclear medicine cameras, which include gamma cameras, Anger cameras and SPECT cameras (SPECT being an acronym for single photon emission computed tomography), must be regularly calibrated to ensure accurate performance. The field of view of a gamma camera is comprised of many pixels, each pixel being determined by a combination of a scintillation detector and photomultiplier tube, or other device to convert incident radiation into electronic signal, and subsequent signal-processing electronics. The pixels of a gamma camera may have inherent differences in response and performance, or “nonuniformity”, since they are dependent on discrete devices, and so they must be normalized to one another such that the same intensity of radiation presented to any pixel of the camera will result in the same intensity of signal (or “counts”) in the corresponding pixel of the final image. This calibration is normally performed using a radiation source which presents a uniform field to all pixels of the camera, said source being commonly known as a “flood source” or “sheet source”.
Current flood sources are generally made of a cast epoxy with a radionuclide or radionuclides evenly dispersed throughout, with this “active element” encased in a rigid plastic housing. These flood sources typically have a weight averaging from about 3.1 kg to 5.5 kg (7 to 12 lbs), depending on manufacturer and model, and the manufacturer generally provides a shielded storage case with rigid sides and a lining of lead or other high-density, high-atomic-number material to block radiation from the source. Storage cases of this style can weigh in excess of 31 kg (70 lbs), and are commonly about 61 cm to 91 cm (2 to 3 feet) high and about 61 cm (2 feet) or more wide by about 7.6 cm to 12.7 cm (3 to 5 inches) thick, to accommodate the rigid flood source. Being tall and excessively heavy for routine carrying, this style of case typically has wheels at the bottom so they may be moved from place to place. Even with wheels, these cases are cumbersome and awkward to ship, handle and move around.
Kalas et al., (US Patent application No. US 20020185613 A1) discloses a method of producing flood sources in which the radionuclide is deposited on the surface of a thin, lightweight substrate and fixed to seal the radionuclide. This “active element” is then encased in an outer housing which is sufficiently rigid to allow for fixed positioning during gamma camera calibration, in order to present a uniform radiation field to the camera. Currently available flood sources of this style have a weight of approximately 1.4 kg (3 lbs), which is more convenient to handle than the heavier cast-epoxy style sources described previously. In Kalas et al., it is disclosed that the thin substrate may be made of a flexible material such as paper, which can be removed from the rigid outer housing and folded or rolled for easier shipment or disposal. However, this style of flood source still requires the rigid outer housing to fix the active element's position in a flat configuration during gamma camera calibration, in order to present a uniform radiation field to the camera. Horst and Menuhr (U.S. Patent Application 20030104178) discuss a method of producing a flood source by printing a radioactive solution on a substrate; including a method of recycling the flood source active element by reprinting on the substrate after the original radioactive printing has decayed. A disadvantage of the methods of Kalas et al., and of Horst and Menuhr is that said methods are based on deposition of the radioactive material on the surface of a substrate. If such a substrate is then flexed, rolled, or folded, the radioactive deposition can develop creasing, cracking, flaking, or other inhomogeneities which render the source unusable for the purpose of gamma camera calibration. Such cracking or flaking may also allow release or dispersion of the radioactivity, contaminating the environment in which it is being used. For these reasons currently available flood sources produced by substrate-deposition methods are encased in a rigid outer encapsulation. In addition to providing a flat geometry of the active element, this rigid capsule protects the active element from creasing, flaking, and otherwise developing structural flaws through repeated handling. Currently available flood sources of this style are generally provided with a rigid-sided shielded storage case of the type described above, and so although the flood source is more convenient to handle than the heavier cast-epoxy style sources, the case remains large, heavy, and unwieldy.
O'Kane et al., (US Patent Application No. 20020060300 A1) discloses a soft-sided shielded storage and transport bag for flood sources, which has a form factor conforming more closely to the dimensions of the flood source, allowing the shielded bag to be of a lighter weight than the hard-sided wheeled cases described above. The latest currently commercially-available version of this shielded bag weighs approximately 14 kg (30 lb), and is manufactured with handles in order that the bag may be carried. An unshielded wheeled case for storage and transport of the bag is an option offered by the manufacturer for users not wishing to carry the about 14 kg (30-lb) bag by the handles.
The dimensions of the active element of a flood source and the level of radioactivity of the source are dictated by the dimensions and specifications of the gamma camera the source is designed to calibrate. In order to provide adequate shielding of the source when not in use, a minimum thickness of shielding material must be used. Since the inner dimensions of the shielding case are dictated by the dimensions of the flood source it is designed to contain, then clearly, for a flood source of given dimensions in a rigid capsule, there is a lower limit to the weight of the shielding case below which said case will not provide shielding adequate for protection of the user when the source is placed in the case.
It accordingly would be desirable to provide flood sources in flexible form factors that can be folded, rolled, etc., to reduce their deployed outer dimensions to a smaller size so that the size and weight of the shielding container can also be reduced. It would also be desirable to provide a radioactive source that can be used when oriented not only on a plane, but also on curved and other non-planar orientations. This flexible radiation source should be durable when flexed, rolled, or folded in order to maintain the integrity and original distribution of radioactivity despite repeated handling.